This invention relates generally to pulp manufacturing processes and equipment, and more particularly to an apparatus for fluffing high consistency pulp in the presence of a gaseous bleaching agent for promoting intimate contact between pulp and bleaching reagent.
Also, more particularly, the present invention relates to a means of manipulating wood pulp fibers within a rotary driven pin type fluffer cyclone to extend the fluffing time and promote uniform mat formation in the presence of a gaseous bleaching agent.
As is known, wood pulp is obtained from the digestion of wood chips, from repulping recycled paper, or from other sources and is commonly processed in pulp and paper mills in slurry form in water. Recently there have been many efforts to use ozone as a bleaching agent for high consistency wood pulp. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and its relatively high cost in the past have limited the development of satisfactory devices.
The primary characteristic of pulp slurries which changes with the consistency of the slurry is the fluidity. Wood pulp in the high consistency ranges (above 18-20% oven-dry consistency) does not have a slurry like character, but is better described as a damp, fibrous solid mass. High consistency pulp can be fluffed, in the same way that dry fibrous solids such as cotton or feathers can be fluffed, to give the pulp a light and porous mass, the inner fibers of which are accessible to a chemical reagent in gaseous form.
The characteristic of compressibility of fluffed pulp, however, makes it difficult to move or transport in conventional solids bulk handling equipment without increasing the bulk density and reducing the porosity (void volume).
To realize fully the advantages of the gas phase reaction in a multistage bleaching of cellulosic fibrous pulp, the comminution of the pulp to produce the fluffed pulp must be of a specific nature so as to produce fragments which independent of their size are of low density, and of porous structure throughout and substantially free from any highly compressed portions, i.e., compacted fibre bundles. Only when this form of comminuted pulp is achieved can the gaseous reactants reach all parts of the comminuted pulp fragments, and thus ensure that the reaction of the gaseous reagent with the fluffed pulp proceeds rapidly and uniformly. The concern for uniformity of contact between the fluffed pulp and the bleaching reagent gas, in the case of ozone bleaching, is fostered by the rapid reduction in the concentration of ozone gas in contact with the fluffed pulp. This reduction is attributable to the extremely fast reaction rate of ozone with wood pulp. Since the reaction rate is concentration dependent, this characteristic increases the non-uniform bleaching results attendant upon the variable permeability of the pulp.
As described hereinabove, the fluffed pulp mass is easily compressed by the action of bulk solids handling equipment to form wads and clumps having much higher density and much lower gas permeability. Bleaching gas flows much more slowly through such wads and clumps and much more rapidly through the wad-to-wad contact areas. The result is overbleached contact areas and underbleached wad cores. Thus, it has been found that bleaching systems which employ conventional bulk materials handling equipment to move high consistency fluffed pulp through a bleaching retention chamber while bleaching it with ozone gas cannot successfully produce uniformly bleached pulp fiber.
Pin shredders and fluffers are used in pulp and paper manufacture and in many other industries for shredding sheet material or fluffing fibrous materials. The size of the particle produced by such a pin shredder depends on several factors such as the size and spacing of the pins, the speed of rotation, retention time, and housing clearance.
An example of such a machine is a fluffer used in high consistency bleaching experiments, and which is described in U.S. Pat. No. 3,725,193 to De Montigny. However, while this machine, and other similar machines, may have operated with varying degrees of success, these machines suffer from a plurality of shortcomings which have detracted from their usefulness.
For example, a disadvantage of using a screen (as suggested in De Montigny) to retain the coarse particles within the housing arises from the fibrous and floccular nature of moist wood pulp. For the flocs to pass through screens, the apertures or slots must be undesirably large, which will result in permitting unfluffed particles of similar size to pass.
Another class of known pin rotor machines used in pulp and paper manufacture consists of a cylindrical housing containing stationary pins on the inside which interleave with pins disposed on a rotor. Such high speed pin rotor machines have operated with varying degrees of success in the low to medium consistency ranges for processing wood pulp, for example as a steam mixer. However, these machines do not operate satisfactorily when processing high consistency pulp, because at high consistency the pulp fibers cling to the base of the stationary pins as they are thrown against them by the rotating pins and by the centrifugal forces of the rotating pulp mass, and the fibers build up to form a plugging condition in the housing, impeding thru flow of the wood pulp being processed.
One possible solution has been proposed which in effect utilizes a vertical constant taper pin rotor cone in a constant tapered conical "cyclone" housing passing the pulp and bleaching gas from top to bottom. This has proved somewhat effective but difficult to control in situations of varying flow rates. There is a tendency to develop a non-uniform pulp mat thickness due to the dynamics of rotation within the constant taper cyclone again resulting in a tendency of non-uniform bleaching at high reaction rates.
The foregoing illustrates limitations known to exist in present machines for fluffing and manipulating high consistency wood pulp. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.